1. Field of the Invention
Embodiments of the invention relate to switching regulators for converting an input voltage to a desired output voltage by ON-OFF control of switching elements, and in particular, to switching regulators that dynamically change the output voltage.
2. Description of the Related Art
FIG. 9 shows an example of construction of a conventionally used common switching regulator. The switching regulator 130 shown in FIG. 9 converts an input voltage Vin to an output voltage Vout by ON-OFF controlling switching elements with pulse width modulation (referred to herein as “PWM”) signals. The switching regulator 130 comprises: a control circuit 131 that controls an output corresponding to the error voltage between a reference voltage Vref and an output detection voltage Vfb that is a detected output voltage Vout; a PWM circuit 132 that outputs a pulse signal of a duty factor (a ratio of an ON period to the total period of ON and OFF periods) according to the output from the control circuit 131; an output circuit 133 that is composed of a pair of switching elements, a P-channel MOSFET (hereinafter referred to as PMOS) QH and an N-channel MOSFET (hereinafter referred to as NMOS) QL, and driving circuits Dr complementarily ON-OFF controlling the switching elements based on PWM signals; an LC smoothing circuit 134 that is composed of an inductor L and a capacitor Cout; and an output voltage setting section 135 that is a voltage divider circuit composed of a resistance R1 and a resistance R2. The output voltage Vout is supplied to a load LD.
The output detection voltage of the output voltage Vout is determined from a reference voltage Vref and the voltage divider ratio in the output voltage setting section 135. The switching regulator 130 shown in FIG. 9 has a variable resistance R1 in the output voltage setting section 135 so that the output voltage can be changed. In order to change setting of the output voltage, the resistance value of the variable resistance R1 must be changed every time the output voltage is to be changed.
FIG. 10 shows an example of construction of a switching regulator that allows the output voltage to be changed responding to the control signal from host devices such as a processor and a power supply managing IC. The switching regulator 140 shown in FIG. 10 installs a resistance R2 for changing the output voltage, the resistance R2 constructing a shunt circuit together with an externally provided output voltage setting section 145. The output voltage Vout is generated and outputted responding to instructions for output voltage setting values from a host 210.
The output voltage setting section 145 comprises parallel connection of the circuits each consisting of series connection of weighted resistance R1i and a switching element Qi, i being an integer from of 0 to m. The output voltage setting section 145 composes a type of digital-to-analogue conversion circuit (referred to herein as a “DAC”) that receives the digital signals from the host 210 to control turning ON and OFF of the switching elements Q1 to Qm and outputs an analogue signal Vfb.
The construction shown in FIG. 10, however, is provided with the DAC 146 that converts instructions such as start and stop from the host 210 and delivers to the switching regulator 140 as well as the digital signal lines from the host 210 to the output voltage setting section 145, resulting in an increased packaging area.
A switching regulator of a digital power supply system is known to cope with the above problem. The switching regulator of the digital power supply system contains a digital communication interface that receives digital signals of instructions directly from the host and allows change of setting according to the instructions within the switching regulator. This type of switching regulator often employs a serial communication interface, which performs communication between the host and the switching, regulator with a small number of terminals and communication lines. The standards for the serial communication interface includes I2C (Inter-Integrated Circuit), SPI (Serial Peripheral Interface), and SM Bus (System Management Bus).
FIG. 11 shows an example of construction of the switching regulator of the digital power supply system. The switching regulator of the digital power supply system 150 of FIG. 11 comprises: a serial communication interface 157 that communicates instructions from a host 220; a DAC 158 controlled through the communication interface 157 and generates a reference voltage Vref; an analogue-to-digital conversion circuit (referred to herein as an “ADC”) 156 that detects an error voltage between an output detection voltage Vfb of an output voltage Vout and the reference voltage Vref and converts the error voltage to a digital error signal e[n]; a controller 151 that performs control operation and outputs a duty factor signal d[n] for instructing a duty factor of a PWM signal from the received digital error signal e[n] under control through the communication interface 157; a digital PWM circuit (hereinafter referred to as a DPWM) 152 that converts the duty factor signal d[n] to a PWM signal with a corresponding duty factor; an output circuit 153 in which turning ON and OFF of switching elements are controlled by the PWM signal; a smoothing circuit 154 that makes a switching output signal smooth; an output voltage setting section 155 in which output voltage setting is controlled to a desired value under the control through the communication interface 157; and a voltage divider R2.
The ADC 156 performs analog-to-digital (hereinafter referred to as “AD”) conversion in every switching period and “n” in the above description represents n-th switching period. There are various types of operational formulas for performing control operation of the duty factor signal d[n], among which a known formula is the digital PID control formula shown by Formula 1 below. In Formula 1, A, B, and C are compensation coefficients in PID (proportional, integral, and differential) control that are to be set to appropriate values corresponding to input and output conditions to stably control the output voltage Vout.[Formula 1]d[n]=d[n−1]+A×e[n]+B×e[n−1]+C×e[n−2]  (1)
The output voltage setting section 155 shown in FIG. 11 is a type of digital-to analogue conversion (DAC) circuit having the same structure as that of the output voltage setting section 145 shown in FIG. 10; the output voltage section 155 comprises parallel connection of the circuits each consisting of series connection of a weighted resistance R1i and a switching element Qi, i being an integer from 0 to m. The switching regulator 150, receiving an output voltage changing instruction from the host 220 through the communication interface 157, changes the reference voltage Vref and controls turning ON and OFF of the switching elements Q0 to Qm to change the divided voltage between the resistance R2 and each of the resistance R10 to R1m in the output voltage setting section 155, thereby controlling change of the output voltage value Vout. This construction having the serial communication interface 157 installed within the switching regulator 150 avoids increase in the number of digital signal lines and the packaging area.